1. Field of the Invention
The present invention relates to a power divider, and more particularly, to a power divider capable of providing high isolation characteristic without using resistors.
2. Description of the Prior Art
Power dividers are wide used in microwave circuits for splitting an input signal into two or more in-phase signals. For example, a Wilkinson power divider or a T-junction power divider are the most commonly used power dividers. In general, the Wilkinson power divider needs an extra resistor for enhancing isolation characteristics. The T-junction power divider does not require using an extra resistor, but has bad isolation characteristics.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a Wilkinson power divider 10 in the prior art. The Wilkinson power divider 10 includes an input port 102, split ports 104, 106, and a resistor 108. The resistance of the resistor 108 is usually twice the impedance of the input port 102. For example, an impedance of the input port 102 is Zo, impedances of the split ports 104, 106 are √{square root over (2)} Zo, and an impedance of the resistor 108 is equal to 2 Zo. In the Wilkinson power divider 10, when an input signal SI enters the input port 102, the input signal SI can be split into equal-phase output signals SO1, SO2 to the split ports 104, 106, and be transmitted to the output ports 110, 112, respectively. However, a resistor element usually has its operating frequency limit, so that when the Wilkinson power divider 10 using a resistor operates at high frequency, parasitic capacitance and inductance effects will occur inducing bad electric characteristics. Please refer to FIG. 2. FIG. 2 is a simulation result of the Wilkinson power divider 10 using an ideal resistor at high frequency, which the input port 102 and the output ports 110, 112 are served as the first port, the second port, and the third port, respectively. Also, the resistor 108 is implemented with a 100 ohm ideal resistor. As can be seen in FIG. 2, though the Wilkinson power divider 10 utilizes the ideal resistor (parasitic inductance effect is removed), the isolation parameter S23 is still poor, only about −8 dB. Thus, even when the resistor 108 is replaced by an ideal resistor, the Wilkinson power divider 10 can also not provide sufficient isolation, and this shows the Wilkinson power divider 10 is really not able to apply to high frequency band.
In addition, as the Wilkinson power divider 10 is operated at high frequency band, the size of the quarter wavelength split ports 104, 106 may become too small. Taking an RO4233 substrate (dielectric coefficient is 3.33) for example, the length of the split ports 104, 106 remains at only 2.4 mm when operating at Ka band. As a result, owing to the short distance between the split ports 104, 106, a large coupling effect will occur. In such a condition, the Wilkinson power divider 10 using an ideal resistor can still not provide enough isolation characteristics.
In short, the T-junction does not need an extra resistor, but can not provide sufficient isolation. Although the conventional Wilkinson power divider can achieve a certain isolation characteristic when operating at low frequency band, enough isolation is not provided when operating at high frequency band.